One of the major problems in periodontal disease and tooth implants is the regeneration of periodontal tissues such as cementum, periodontal ligament and alveolar bone. Considerable knowledge as accumulated in recent years related to bone regeneration, however, our understanding of cementum and periodontal ligament regeneration is still obscure. It is the long term objective of this study to understand the cellular and molecular mechanisms underlying cementum formation and root attachment to periodontal ligament in the health periodontium. In this application, we focus on cementum formation and in particular the role of Hertwig's Epithelial Root Sheath (HERS) cells in this process. Although the role of HERS cells in cementum formation is widely accepted, the precise function and fate of these cells remains controversial. We propose to test the hypothesis that HERS cells synthesize and secrete enamel- related proteins which form intermediate and acellular cementum and then these cells undergo epithelial-mesenchymal transformation to become cementoblasts to form cellular cementum upon root surfaces. This project will take advantage of an Immortomouse-derived HERS cell line established in our laboratory. Experiments are designed to first determine if the enamel-related proteins expressed by the HERS cells are necessary for cementogenesis in vitro. The regulatory mechanisms leading to epithelial-mesenchymal transformation and the nature of the cementum extracellular matrix formed will be investigated. Finally, we will examine if the events determined using the HERS cells in vitro also take place in vivo using the mouse animal model. The data generated in this study will provide valuable information related to root formation and will help to clarify the role of HERS in cementum formation and possible ways to improve the diagnosis, treatment and prognosis of tooth loss due to aging or periodontal diseases.